Effect of the Electric Double Layer (EDL) in Multicomponent Electrolyte Reduction and Solid Electrolyte Interphase (SEI) Formation in Lithium Batteries

Electrolytes, consisting of salts, solvents, and additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium(Li)-ion batteries. However, the electric double layer (EDL) structure near charged surfaces is still unsolved, despite its importance in dictating the species being reduced for SEI formation near a negative electrode. In this work, a newly developed model was used to illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. Both electrolytes have fluoroethylene carbonate (FEC) as a common additive to form the beneficial F-containing SEI component (e.g., LiF). However, the role of FEC drastically differs in these electrolytes. FEC is an effective SEI modifier for the carbonate-based electrolyte by being the only F-containing species entering the EDL and being reduced, as the anion (PF₆^-) will not enter the EDL. For the ether-based electrolyte, both the anion (TFSI^-) and FEC can enter the EDL and be reduced. The competition of the two species within the EDL due to the surface charge and temperature leads to a unique temperature effect observed in prior experiments: the FEC additive is more effective in modulating SEI components at a low temperature (-40 °C) than at room temperature (20 °C) in the ether-based electrolyte. These collective quantitative agreements with experiments emphasize the importance of incorporating the effect of the EDL in multicomponent electrolyte reduction reactions in simulations/experiments to predict/control the formation of the SEI layer.